Understanding Audio Amplifiers Specs

What makes a good home theater power amplifier?

The best way to evaluate amplifier performance is to listen to it while driving your speaker system within your home theater room. But even if this is not possible, an amplifier specs sheet gives a good indication of the amplifier's ability to deliver a great sound.

Factors like amplifier power rating, distortion levels, amplifier dynamic headroom, and signal-to-noise ratio tell a lot about a power amplifier's ability to deliver adequate audio power when needed and the level of sound quality.

Having an understanding of audio amplifiers specs and in particular what makes good amplifiers is thus very important when selecting an amplifier, whether that being a 'separate' high-end multi-channel audio power amplifier or a budget-class AV receiver.

What makes good audio amplifiers? Evaluating amplifier specifications

The simple answer is good sound! This should be your main determining factor - implying extensive listening test before choosing your home theater receiver or amplifier, but...

The reality is very few will ever have the possibility to listen to as many power amplifiers they would like to when making a purchase, especially when buying online. Yet for the informed, an amplifier specs sheet can tell a lot about an amplifier ability to deliver good sound. It is important to realize however that there is no single spec that can define a home theater receiver or audio amplifier's ability to deliver a great sound. So do not single out one spec - no matter how important it is to you - without taking into account the rest, whether that being amplifier power rating, noise level,
etc.

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Many do the mistake of assessing an audio amplifier simply on its power capability. Yet power is one of the most misleading and most abused amplifier indicators by equipment manufacturers despite regulations mandating specific reporting for amplifier power.

As such, having an understanding of the most important amplifier specs and how best to interpret such specs is crucial to avoid costly mistakes during a purchase while ensuring your amplifier will be up to your listening habits.

Amplifier power rating

Power is expressed in watts. But in the case of audio amplifiers or AV receivers, there is a lot more than just watts; a typical amplifier power rating is defined as say '80W/Ch RMS 20Hz-20kHz, 8 Ohms 0.09% THD.'

This means that the amplifier is capable of delivering 80 watts average power per channel into an 8 Ohm speaker load at a maximum 0.09% total harmonic distortion over the specified frequency range when driven by a sinusoidal test tone - i.e. a single frequency swept over the entire frequency range.

This format was originally mandated by the US Federal Trade Commission and as such, serves to provide a rather stable reference when comparing different amplifiers from different brands. Note that this is not the case with powered subwoofers; there are no such regulations with built-in power amplifiers, thus explaining why powered subwoofer
ratings are often exaggerated in the way they are reported by their respective manufacturers.

We said that the reported W/Ch RMS represents the average power per channel. While many refer to power as RMS power, when it comes to power, it is more correct to refer to average power. RMS (root-mean-square) makes sense only with variables that have both negative and positive values as in the case of an alternating voltage. There
is no such thing as negative power; hence, the 'root' and the 'square' of the RMS process would only serve to extract the sign out of a number. This means that RMS power is actually the average power derived using the RMS voltage values across the speaker load.

In their attempt to come up with even bigger numbers, some manufactures also specify peak power; this corresponds to the audio amplifier peak power output as derived from the amplifier peak output voltage when driving a loud, transient high point. The actual relation between peak power and average power depends very much on the type
of signal. For example, in the case of a typical pink noise audio test signal (i.e. one where each octave carries an equal amount of noise power) with a 6dB crest factor, peak power is equal to four times the average power.

While peak power per se is not as important as the average power, it can serve as a valid indication of an audio amplifier's ability to respond to musical peaks say during an orchestra playback and short duration special loud effects during movie watching. To what extent an amplifier is capable of responding to such loud transients depends
on the amplifier dynamic headroom.

Dynamic headroom - expressed in dB, represents the reserve transient power that an amplifier is capable of delivering for the short period of time that the transient lasts. This ability of a home theater receiver or audio amplifier to deliver bursts of power is important for an accurate sound reproduction. An amplifier rated at 100 watts
average power but that can deliver 400 watts peaks is said to have 6dB headroom.

An amplifier that does not have the ability to reproduce sufficient high peaks will compress those peaks to fit within its headroom; the result is a significantly reduced sonic impact.

Amplifier sound quality

Directly related with amplifier output is sound quality. While amplifier power per se is important as it is what will enable you to match the loudness in the room to your listening habits, yet it is more important to have better sound than more sound. You will better enjoy a 50W amplifier of superior sound quality against a 100W
amplifier of poor sound.

In addition, as we will further explain in this discussion, doubling amplifier power will not double the resultant loudness, or sound volume in the room. You need ten times as much amplifier power for your ears to perceive sound twice as loud.

So... do not let audio amplifier manufacturers play the power number game on you; ensure to take into account also the other important specs relating to sound quality.

In the case of audio amplifiers, the main indicators that describe sound quality are distortion or total harmonic distortion (THD) expressed as percentage THD, and signal-to-noise ratio or S/N level, expressed in dB. Some would also include the amplifier dynamic headroom referred to above as part of an amplifier sound quality
indicator in that an amplifier with insufficient headroom would fail to deliver the highs in the way these were intended to be.

Distortion Levels

Distortion is a measure of the amount by which an audio amplifier output is distorted, or altered with respect to the input signal. Distortion levels below 1% are not considered significant, nor would you perceive any difference between say an amplifier with a rated 0.05% THD and one with a rated 0.1% THD. At the same time, keep in mind
that high levels of distortion can make an amplifier unlistenable; an amplifier with a rated 10% THD at full power would deliver mediocre sound quality not only at full power but possibly even at lower volume levels.

The issue with total harmonic distortion arises as higher levels of distortions affect the higher order harmonics even more. Harmonics refer to the higher order tones - or frequency multiples of the original note; these are what differentiate say a piano note from the same note played on a different instrument.

Note: While THD ratings would give you an indication of the distortion levels present, yet it would not give you any indication of the nature of the distortion. Some flagship amplifier models give the distortion levels at different order-harmonics - with the most common being the 3rd and 5th harmonics. This
gives a better picture than just a total level but for the purist, even this is not enough. The only way is to trust your ears; there is no substitute to taking the time to listen when making an amplifier purchase - inasmuch as you have to listen to make a speaker purchase.

Amplifier Class

Directly related with total harmonic distortion is the amplifier class - A, B, AB , or D. Amplifier class defines the type of amplification used in the amplifier output stage and as such impacts the distortion levels produced by a power audio amplifier. In very simple terms, a power amplifier output stage consists of two main sub
power stages, one to amplify the negative voltage signal while the other to amplify the positive part.

A Class A design keeps both stages 'on' at all times; it is the one that delivers the best sound but also the most inefficient - with an efficiency that can range from approximately 10% to 20% (excluding other inefficiencies in the rest of the amplifier stages and power supply.)

It is therefore the one that generates the most heat as maximum current is present all the time irrespective of the audio signal level, thus leading to a highly inefficient design in order to deliver the best sound.

Class B pulls and pushes - hence the term push-pull stage - current between the two sub-power stages so that as one is on, the other is off. This leads to a more efficient design - with an efficiency that can range from about 50% to 75%. But this design also introduces a much higher level of harmonic distortion due to this continuous
on/off process.

Class AB is a hybrid between the two in that it leaves both amplification stages 'on' only for the smaller part of the signal - thus working in class A for low level signals while switching to class B operation when handling larger amplitude signals.

This leads to the best of both worlds in that you get the best sound for lower audio signals - which in most cases constitute the larger portion of an audio or music soundtrack, and the improved efficiency of class B operation when needed most - i.e. when handling high signal levels.

Class D amplifiers convert the input signal into a sequence of higher voltage output pulses; for this reason, these are often mistakenly referred to as digital amplifiers. The output pulses are then fed to a low-pass passive filter before being feed to the speaker to remove unwanted frequency components from the signal. The resulting
filtered signal is then an amplified replica of the input.

These are the most efficient audio amplifiers (with efficiencies in excess of 90%) but in terms of sound quality, these amplifiers are often at the lower end of the scale. For this purpose, Class D audio amplifiers are mainly used in lower-end HTiBs and in powered subwoofers, though some high-end designs are starting to appear.

Noise Levels

Signal-to-noise (S/N) ratio is a measure of the sound signal level with respect to background noise. The larger the ratio, the better is the separation between the sound signal - whether that being music, a movie soundtrack, etc., and the background noise level. Background noise in an amplifier can be of various origins and include among
others, hum pickup, noise on the AC supply, and even electrical noise caused by the chaotic movement of the free electrons within the various electronic components constituting the amplifier itself.

An audio amplifier with a rated S/N of 90dB has excellent sound to noise ratio. Noise levels become noticeable mainly at lower sound signals; for example, with an amplifier having an S/N ratio of say 65db, noise levels at high volume amplifier settings may become too noticeable and even annoying during quite passages of music and dialogue.

Amplifier Power - Watch out for manufacturers' trick!

Be careful with the way manufacturers report audio amplifiers and AV receivers power ratings. Despite the mandated format by the US FTC, many manufacturers still manage to find ways of exaggerating power ratings.

One very common trick is to reduce the lower frequency range from 20Hz to 40Hz - just an apparent minimal reduction but one that would greatly reduce the demand on the amplifier imposed by ultra-low bass content.

Others may specify peak power with only one channel driven at a time - thus easing the load on what would be effectively an underrated amp power supply. Look for the term 'all channels driver'; unless it is specifically written in the amplifier specs, there is no guarantee that the amplifier will be able to run all channels at their maximum
rated average power.

At times, in AV receivers and multi-channel surround sound audio amplifiers, instead of all channels driven, you find the term 2 channels driven; this is not enough when dealing with multi-channel audio amplifiers for surround sound. And yes, check the audio amplifier weight - a big enough power supply capable of delivering the full rated
power output on all channels is always big and heavy.

In some case, manufacturers would specify lower speaker impedance than 8 Ohms - with 6 Ohms being the most common alternative.

Lower impedance speakers draw more current, thus enabling the AV receiver or amp manufacturer to specify a higher power output. An amplifier capable of a maximum 100W average power in a 6 Ohm load may only deliver 75W average power in an 8 Ohm speaker load. For the same audio amplifier to deliver 100W into an 8 Ohm load, it will require among
others a bigger power supply and a higher power amplifier output stage.

In other words, unless the manufacturer tells you the power output at both 8 Ohms and say 6 Ohms, the 6 Ohm power rating alone would not tell you how that amplifier will behave with standard 8 Ohm home theater speakers. On the other hand, having both power out ratings specified will tell you if the amplifier is capable of delivering more
current and if so, at what levels of distortions.

Note: We discuss the subject of audio amplifier power in substantial detail in our article: Amplifier Power - Matching the amplifier power output to your expected level of loudness.